Improved mechanical shift, pneumatic assist pilot valve

Abstract

 A pneumatic assist valve receives constant air pressure from supply air to provide the pneumatic assist to shift the pilot valve, thereby eliminating false signals acting on the trip rod. The design also ensures that the pilot valve has completely shifted before diaphragm reversal occurs.
There is disclosed a mechanical shift pneumatic assisted pilot valve for a reciprocating function comprising a reciprocating piston disposed in a bore intermediate a first and a second reciprocating element and being provided with a means at one end for directly contacting said first reciprocating element in one operating position and a pneumatic piston at another end, said pneumatic piston being further provided with a means for contacting said second reciprocating element in a second operating position; and said pneumatic piston being a stepped piston having a lesser diameter constantly pressurised in one biasing direction and a greater diameter alternately pressurised in an opposite biasing direction in response to mechanical shift of said pneumatic piston effected by said means for contacting said second reciprocating element said mechanical shift further effecting reversal of direction of said first and second reciprocating elements.

Description

[0001] This invention relates generally to mechanical shift, pneumatic assist valves and more particularly to a mechanical shift pneumatic assist valve for diaphragm pumps which use a separate pilot valve to provide a positive signal (either on or off to the major air distribution valve).

[0002] Here there is disclosed an improvement of the device described in US-A-4,854,832 assigned to the Aro Corporation. The prior art device significantly reduced the possibility of motor stall by providing a positive signal (either on or off) to the major air distribution valve. This was accomplished by adding a separate valve (pilot) which was not connected to the diaphragm rod. Actuation of the valve was accomplished by mechanically pushing the valve to the trip point with the diaphragm washer attached to the diaphragm connecting rod causing the major valve to shift. As pressure built up in the diaphragm air chamber it also acts on the end of the pilot rod (area) and forced it to the end of its stroke. Air pressure holds it in this position until the diaphragm washer pushes it in the opposite direction. As long as the pilot rod was in either extreme position, a signal is always present to the major valve.

[0003] Other designs, which incorporate the "pilot" on the diaphragm connecting rod, shut the signal off to the major valve after the diaphragm changes direction.

[0004] Occasionally an air pressure spike occurs in the diaphragm air chamber which is being exhausted. The spike occurs when there is an unusually rapid reversal of the diaphragms due to a malfunctioning check valve or a large volume of air trapped in one or both air caps or a restriction in the exhaust. If this pressure spike exceeds the pressure of the incoming air of the chamber being pressurised to pneumatically assist the trip rod, the spike can cause the trip rod to back up. Depending on the pump speed, operating pressure and severity of any one of the above conditions, the pump may begin to rapidly short stroke because the trip rod is oscillating back and forth around the trip point and out of sync with the diaphragm rod. Occasionally this condition results in a motor stall.

[0005] According to the present invention there is provided, a mechanical shift pneumatic assisted pilot valve for a reciprocating function comprising a reciprocating piston disposed in a bore intermediate a first and a second reciprocating element and being provided with a means at one end for directly contacting said first reciprocating element in one operating position and a pneumatic piston at another end, said pneumatic piston being further provided with a means for contacting said second reciprocating element in a second operating position; and said pneumatic piston being a stepped piston having a lesser diameter constantly pressurised in one biasing direction and a greater diameter alternately pressurised in an opposite biasing direction in response to mechanical shift of said pneumatic piston effected by said means for contacting said second reciprocating element said mechanical shift further effecting reversal of direction of said first and second reciprocating elements.

[0006] For a better understanding of the invention and to show how the same may be carried into effect, reference will now be and 9b. Both valves are located in the same cavity 12 which is pressurised with supply air. The power piston 4 and the pilot piston 7 are differential pistons. Air pressure acting on the small diameters of the pistons will force the pistons to the left when a pilot signal is not present in two chambers 10 and 11. The area ratio from the large diameter to the small diameter is approximately 2:1. When the pilot signal is present in the chambers 10 and 11 the pistons are forced to the right as shown in Figs. 5 and 6.

[0007] In Fig. 4 the spool 1 of the main valve is shown in its extreme left position, as is the pilot piston 7 in Fig. 3. Air in the cavity 12 flows through an orifice 13 created between the spool 1 and the valve block 2 through a port 14 in the valve plate 3. The air impinging on the upper surface of the check valve 5a forces it to seat and seal off an exhaust port 15. The air flow deforms the lips of the elastomeric check valve as shown in Fig. 4. Air flows around the check valve into a port 17 and into a diaphragm chamber 18. Air pressure acting on a diaphragm 19 forces it to the right expelling fluid from a fluid chamber 20 through an outlet check valve 50 (see Fig. 1).

[0008] Operation of the fluid check valves control movement of fluid in and out of the fluid chambers causing them to function as single acting pumps. By connecting the two chambers through external manifolds 51 output flow from the pump becomes relatively constant.

[0009] At the same time as the chamber 18 is filling, the air above check valve 5b has been exhausted through an orifice 21, a port 22 and into an exhaust cavity 23. This action causes a pressure differential to occur between chambers 24 and 25. The lips of check valve 5b relax against the wall of the chamber 25. As air begins to flow from an air chamber 26 through a port 27, it forces check valve 5b to move upward and seat against the valve plate 3, seal off a port 28 and open a port 16. Exhaust air is dumped into the cavity 23.

[0010] The diaphragm 19 is connected to a diaphragm 29 through a shaft 30 which causes them to reciprocate together. As the diaphragm 19 traverses to the right the diaphragm 29 evacuates a fluid chamber 31 which causes fluid to flow into fluid chamber 31 through an inlet check valve 55. As the diaphragm assembly approaches the end of the stroke, a diaphragm washer 33 pushes the actuator pin 9a to the right. The pin in turn pushes the pilot piston 7 to the right to the position shown in Fig. 5. An O-ring 35 is engaged in bore of a sleeve 34 and an O-ring 36 exits the bore to allow air to flow from the air cavity 12 through a port 37 in the pilot piston 7 and into the chamber 10. Air pressure acting on the large diameter of the pilot piston 7 causes the piston to shift to the right.

[0011] The air that flows into the chamber 10 also flows into the chamber 11 through a passage 38 which connects the two bores. When the pressure reaches approximately 50% of the supply pressure, the power piston 4 shifts the spool 1 to the position shown in Fig. 6. Air being supplied to the chamber 18 is shut off and the passage 38 is exhausted through an orifice 41. This causes check valve 5a to shift connecting the diaphragm chamber 18 to the exhaust port 15. At the same time the air chamber 26 is connected to supply air through an orifice 40 and the ports 28 and 27. The air pressure acting on the diaphragm 29 causes the diaphragms to reverse direction expelling fluid from the fluid chamber 31 through an outlet check valve 56 while the diaphragm 19 evacuates the fluid chamber 20 to draw fluid into fluid chamber 20.

[0012] As the diaphragm 19 approaches the end of its stroke, a diaphragm washer 39 pushes the actuator pin 9b. The motion is transmitted through the pushrod 8 to the pilot piston 7 moving it to the trip point shown in Fig. 2. The O-ring 36 re-enters the bore in the sleeve 34 and seals off the air supply to the chambers 10 and 11. The O-ring 35 exits the bore to connect the chambers 10 and 11 to the port 37 in the pilot piston 7. The air from the two chambers flows through the port 22 into the exhaust cavity 23. Air in the air cavity 12 acting on the small diameters of the pistons 4 and 7 forces both to the left as shown in Fig. 3. The power piston 4 will pull the spool 1 to the left to begin a new cycle as shown in Fig. 4.

Claims

1. A mechanical shift pneumatic assisted pilot valve for a reciprocating function comprising:
   a reciprocating piston disposed in a bore intermediate a first and a second reciprocating element and being provided with a means at one end for directly contacting said first reciprocating element in one operating position and a pneumatic piston at another end, said pneumatic piston being further provided with a means for contacting said second reciprocating element in a second operating position; and said pneumatic piston being a stepped piston having a lesser diameter constantly pressurised in one biasing direction and a greater diameter, characterised in that said greater diameter of said piston is alternately pressurised in an opposite biasing direction in response to mechanical shift of said pneumatic piston effected by said means for contacting said second reciprocating element, said mechanical shift further effecting reversal of direction of said first and second reciprocating elements.

2. A pilot valve according to claim 1, wherein
   said first and second reciprocating elements further comprise pumping elements.

3. A pilot valve according to claim 2, wherein
   said pumping elements are pump diaphragms.

4. A pilot valve according to claim 1, 2 or 3, wherein
   said means at one end for directly contacting said first reciprocating element in one operating position comprises a contact pin of minimum structural diameter projecting into a pressurised operating cavity of a pumping element so as to minimise the cavity pressure effect on said contact pin and said reciprocating piston.

5. A pilot valve according to any one of the preceding claims, wherein
   said means for directly contacting said second reciprocating element in a second operating position comprises a second contact pin of minimum structural diameter projecting into a pressurised operating cavity of a pumping element thereby minimising the cavity pressure effect on said second contact pin and said pneumatic piston.

6. A pilot valve according to any one of the preceding claims, wherein
   said pneumatic piston further comprises a stepped piston having a greater diameter face alternately exposed to pressure fluid to effect longitudinal translation of said pneumatic piston in response to said pneumatic piston being displaced by said second contact pin in a longitudinal direction.

7. A pilot valve according to claim 6, wherein
   said pneumatic piston is disposed in a stepped bore having a major diameter and a minor diameter corresponding to and co-operating with a major diameter and a minor diameter of said pneumatic piston.

8. A pilot valve according to claim 7, wherein
   said stepped bore is sealed at its said major diameter end, open to a constant source of pressure fluid at its minor diameter end, and vented intermediate its major and minor ends.

9. A pilot valve according to claim 8, wherein
   said pneumatic piston is further provided with means for alternately effecting flow of pressure fluid from said constant source of pressure fluid to said major diameter end and to vent in response to mechanical shift of said pneumatic piston.

10. A pilot valve according to claim 9, wherein
   said means for alternately effecting flow of pressure fluid from said constant source of pressure fluid to said major diameter end and to vent in response to mechanical shift of said pneumatic piston further comprises a valve on said pneumatic piston minor diameter and a passage interconnecting said valve and said major end of said pneumatic piston.

Drawing